The invention relates to satellite communications systems, and more particularly, to such systems for communicating between mobile subscribers and at least two ground stations connected to a communications network such as the Public Switched Telephone Network (PSTN) or the Internet.
It is known to communicate between a plurality of mobile terminals and a central ground station via an orbiting relay satellite using multiple transmission and reception beams between the satellite and the mobile terminals, while locating the processing for controlling the multiple beams, known as an adaptive beamformer, at the central station rather than on board the satellite. A considerable reduction in complexity of the orbiting satellite results when practicing this art. The technique relies upon communication of multiple signals between the central ground station and the satellite in such a way that their relative phase and amplitude, i.e. coherency, is preserved. One coherent transmission scheme is to sample each of the multiple signals at or above the Nyquist rate and then to form a-high speed time-multiplex of the sampled signals. Known signals may be included in the time-multiplexed stream for facilitating time and frequency synchronization at the satellite.
The reverse link from satellite to central station also preferably uses a high speed time-multiplex of signals received at the satellite by different elements of a multi-element antenna to preserve relative coherency, thus permitting ground-based beamforming for reception as well as transmission. Relative coherency is preserved by time multiplexing through the use of a first time multiplexer for time-multiplexing the real (Inphase or I) parts of a sampled complex signal waveform and a second time-multiplexer, synchronized with the first time-multiplexer, for time-multiplexing the corresponding imaginary (Quadrature or Q) parts of the sampled complex signal waveform, which technique will be referred to as quadrature time division multiplexing.
Mayfield et al., in U.S. Pat. No. 5,903,549 entitled “Ground based beamforming utilizing synchronized code division multiplexing” teaches using CDMA feeder link transmission to maintain the desired coherency between antenna array element signals, and the '549 patent is incorporated by reference herein.
In prior art systems, a single central station (ground station) relays signals through one or more satellites, thereby providing service to a number of mobile terminals. However, using a single ground station is not optimal in certain communication systems. Because one or more satellites provide coverage over a significant geographic area, a single ground station may be geographically distant from the end destination of a mobile terminals user's call. Thus, the routing of the call from the ground station to the end destination may be subject to long-distance charges. Multiple ground stations arranged with sufficient geographic separation minimizes the maximum toll charges required to complete a given mobile terminal user's call.
A further disadvantage arises from frequency re-use limitations inherent in single-ground station systems. Multiple ground stations that are sufficiently separated geographically may reuse the same frequency spectrum to communicate with the one or more satellites. This is possible because the satellites can easily distinguish between multiple signals in the same frequency spectrum provided there is adequate spatial diversity between the originating signal sources.
However, there remains a need for a communications system in which multiple ground stations advantageously arranged in a given geographic region and each employing beamforming techniques can cooperatively relay signals through one or more supporting satellites to a plurality of mobile terminal users.